Because of the reduced component sizes in circuits, there is a tendency in electronic circuits to reduce supply voltages used in the circuits. In conjunction with this reduced supply voltage, the maximum possible signal level is also reduced and a signal/noise ratio is impaired.
In the case of switched-capacitor sigma-delta modulator circuits, which are used, for example, in broadband data-transmission systems, the maximum possible signal level depends, for example, on a reference voltage that is limited in turn by the supply voltage. For a favorable signal-noise ratio, the chosen reference voltage should be as high as possible without saturation of the signal levels occurring at the nodes of the circuit. Alternatively, there is the possibility of achieving a specified signal/noise ratio by reducing the noise level through an appropriately modified design of the circuit.
One problem of the procedure of adapting the reference voltage to the lower supply voltage and achieving the specified signal/noise ratio by means of a reduction in the noise level is an increased current consumption of the circuit. For example, a reduction in the noise level by a factor of 2, i.e., by 3 dB, is associated with an approximate doubling of the power consumption.
In addition, in switched-capacitor sigma-delta modulator circuits, a thermal noise that is proportional to the reciprocal value of a circuit capacitance has to be reduced to the same extent, i.e., an increase in capacitances is necessary. The increase in capacitances is associated, in typical semiconductor circuits, with an increased space requirement on a semiconductor chip used to implement the circuit.
A further possibility for achieving the specified signal/noise ratio is to insert a feed-forward loop into the sigma-delta modulator circuit in addition to a loop filter and a feedback loop. Normally, in the case of sigma-delta modulator circuits, the loop filter comprises, for the purpose of noise shaping, at least one integrator or integrator means or a plurality of integrator or integrator means connected in series. An input signal for a quantization circuit is derived from an output signal of a final integrator or integrator means in the series connection. The feedback loop is designed to feed back an output signal of the quantization circuit to the signal inputs of the integrator or integrator means.
In this connection, the feed-forward loop serves the purpose of minimizing a signal component of an original input signal of the sigma-delta modulator circuit in the loop filter by forward-feeding an input signal of the sigma-delta modulator circuit to summation points or nodes of the sigma-delta modulator circuit. This enables the signal amplitudes at the summation points or nodes to be reduced at the signal input of integrator or integrator means in the loop filter so that a higher value can be chosen for the reference voltage without a saturation of the signals occurring at the respective nodes in the event of full modulation. Furthermore, simplified amplifier systems, in particular single-stage amplifier systems, may be used in the integrator or integrator means.
Forward-feeding the input signals of the sigma-delta modulator circuit requires further signal additions. In analog circuits, such as, for example, the loop filter of a sigma-delta modulator, the addition of signals is normally implemented with the aid of active blocks (e.g., operational amplifiers). An active block provided exclusively for the purpose of adding signals is necessary, in particular, for adding at the signal output of a final integrator or integrator means of the loop filter, i.e., at the signal input of the quantization circuit. One problem of this procedure is consequently an increased area requirement of the circuit arrangement on the semiconductor chip and also an increased power consumption.